What is the fastest way to create a 90° joint?
The spin tool is well suited for creating joins:


Spin Alt-R it works best (expected results) in orthographic mode. The selected parts of the mesh will be rotated around the 3D cursor. The 3D cursor should precisely set vie the properties panel N.

The last operator settings allow to adjust the angle and steps:

If you focus more on modelling speed than accuracy the the bend tool could also be useful: How does the bend tool work?